The long-term goal is to understand neuromusculoskeletal dynamics of posture and locomotion. Despite the ability to quantify and simulate motor behaviors, one currently has little understanding of how the nervous system coordinates complex muscle activation patterns to produce motor functions during natural behaviors. Without this basic understanding, one cannot predict the impact of disordered muscle patterns on motor function in neurologically impaired individuals. This proposal explores the exciting new concept that complex muscle activation patterns can be decomposed into a few functional muscle groupings, termed muscle synergies. This simple and functional partitioning has important implications for the design of neural prostheses, therapeutic interventions to restore motor control, and for understanding the structure and function of neuronal mechanisms for movement. The objective of this proposal is to identify muscle synergies and their biomechanical functions in the cat hindlimb during postural responses in normal and neurologically impaired animals. The PIs hypothesize that a limited number of muscle synergies can produce the complex muscle activation patterns observed during postural responses, and that each muscle synergy produces a specific, task-related mechanical function. The novel approach is to integrate the analysis of detailed experimental data with computer simulations to understand how muscle synergies provide the nervous system with 'building blocks' for generating natural motor behaviors. The cat is an ideal model for these studies because (1) it allows examining muscle synergies in the same animal in a variety postural tasks, both pre- and post-neurological impairment, and (2) it allows to directly test the validity of the model with in vivo muscle stimulation experiments. The results will establish a new theoretical framework for understanding how the nervous system implements control of task-level variables at he level of muscle activations. One of the major innovations of this proposal is a novel method of synergy identification that allows for breaking complex objects into meaningful parts. Using this new framework the PIs will evaluate how muscle synergies change with specific neurological impairments and the resulting effect on motor function. In Aim 1 they will identify muscle synergies during balance cats, in intact and neurologically impaired cats (spinal cord transection or large fiber peripheral neuropathy), and determine their relationship to ground reaction forces. In Aim 2 the PIs will implement and use a 3-D musculoskeletal model of the cat hindlimb to define static biomechanical constraints on muscle synergy organization for ground reaction force production. This work forms a theoretical basis for the functional interpretation of complex muscle patterns that underlie natural motor behavior. It provides a critical link between muscle coordination and function that will drive the development of practical solutions to restore motor function in neurologically impaired individuals such as neural repair, neural prosthetics, and neural rehabilitation.